Synchronous copy and asynchronous copy are known as methods of remote copy for copying data recorded in a storage device into another storage device for backup. In the synchronous copy, a copy source device stores data for which a write is requested from a host device, and creates a copy of the data in a copy destination device, and reports completion of the write to the host device after completing the copy. On the other hand, in the asynchronous copy, the copy source device reports the completion of the write to the host device when storing the data for which the write is requested, and creates a copy of the data in the copy destination device at an asynchronous time after the report.
Although data is always duplicated, the synchronous copy has a characteristic that a round-trip time (RTT) increases as the distance between devices increases, deteriorating responsivity to writing. Hence, the asynchronous copy is suitable for application in which the copy destination device is located at a long distance, considering a large scale disaster that affects a wide area.
In this connection, there is proposed a system in which storage devices are located at three sites, and synchronous copy is performed from a first storage device to a second storage device, and asynchronous copy is performed from the first storage device to a third storage device. According to this system, not only the possibility of losing data is reduced by the increase of backup sites, but the third storage device can be located at a long distance without deteriorating the responsivity of the first storage device to the host device.
Also, the next method capable of copying with the order maintained is proposed as an example of asynchronous copy technology. In this method, a copy source device stores data of a first storage medium in a first record buffer, and when this buffer becomes full, transmits the data in the buffer to a copy destination device at a time. The copy destination device receives and stores the data in a second record buffer, and when reception of the data is completed, spreads the data in the buffer to a second storage medium.
See, for example, Japanese Laid-open Patent Publication Nos. 2003-122509 and 2006-260292.
Here, it is assumed that asynchronous copy is performed with the order maintained, by applying the above method using the record buffers to the above system including the first to third storage devices located at three sites. According to this configuration, when the first storage device stops operating due to failure or the like, the second storage device can take over in/out (I/O) processing according to a request from the host device. However, when the first storage device stops operating, it is possible that the data stored in the second storage device is not identical with the data stored in the third storage device. It is possible that the order of writes is broken if these data are made identical in order to start the asynchronous copy. In order to make these data identical with each other while maintaining their order, the copy destination of the synchronous copy needs to be equipped with a mechanism for maintaining the order of the data for which the asynchronous copy has not been executed yet.